Project Summary/Abstract The overall goals of this project are to 1) investigate the etiology of the unique neuropathology associated with arginase 1 (A1) deficiency, a disorder of the urea cycle, and 2) to extensively demonstrate that AAV-based hepatic gene therapy is effective in preventing the features of this disorder as a prelude to a clinical trial. A1 deficiency results in chronic hyperargininemia characterized by progressive mental impairment, spasticity, and growth retardation, with only periodic episodes of hyperammonemia. Recent and preliminary findings from our laboratory with the A1-deficient mouse have demonstrated substantial anatomical, ultrastructural and electro- physiological differences between knockouts and wild types. A1 deficiency led to decreased intrinsic excita- bility, altered functional synaptic transmission, decreased dendritic arborization, dysmyelination and decreased synaptic density. The most likely mechanism causing these neuronal abnormalities is hyperarginine- or guani- dino compound-mediated dysfunction of neurons and oligodendrocytes. Controlling plasma arginine and guani- dino compounds following administration of liver-specific AAV-based gene therapy resulted in much of these measures being substantially improved. The finding abnormalities at the neuron, synapse, myelin, and circuit level have begun to elucidate the functional deficits in A1 deficiency. The identification of the proximate toxin and mechanism of neurodysfunction will open doors to potential pharmacological interventions for A1 deficiency in addition to gene therapy, and may open avenues to new therapies for other disorders where dysmyelination is a feature. Preliminary data: Our research group has (amongst other findings): 1) constructed and characterized the A1-deficient mouse; 2) demonstrated long-term survival with liver-specific recombinant AAV; 3) demonstrated that only low-level ureagenesis is necessary for survival; 4) shown that gene therapy- treated A1 knockout mice lack gross nervous system abnormalities; 5) shown that peripheral metabolism results in control of circulating plasma arginine; and 6) shown that loss of A1 gene expression results in abnormalities of intrinsic excitability, synapse type and number, myelination and the dendritic arbor of neurons. In Aim 1, the hypothesis that oligodendrocyte dysfunction and death result in dysmyelination and is in part the cause of neuronal dysfunction in A1 deficiency will be tested. In Aim 2, the hypothesis that elevated guanidino compounds can induce alterations in intrinsic excitability and synaptic transmission that are similar to those seen in A1 deficient animals will be tested. In Aim 3, it will be determined if A1 deficiency causes an imbalance in excitation and inhibition, and if this inequality is mainly through effects on perisomatic inhibition. Completion of these studies will provide a greater understanding of and the mechanism(s) behind the alterations in the brain, neurons, and synapses in A1 deficiency and hyperargininemia while demonstrating the efficacy of hepatic A1 gene therapy in preventing these abnormalities, providing strong evidence for this therapy as a prelude to its clinical adoption in patients with this progressive neurological disorder.